So the ST is better? If I go ST, I will need to buy a PCIe-1x wifi card, but that isn't really that expensive.

I have no preference for ASUS or any other company, so whatever you think will be best for my uses. I also am only limited within reason on budget(for me, less than $500), so that's not really a concern I think.

I'm liking the Essense a lot right now. Getting rid of the stupid 6.3-3.5 adapter will be a huge bonus, and the built in amp holds a lot of appeal to me as well.

I have heard replacing the cables on the HD555 really help the sound, but this obviously would be a step after the sound card. Do you have any input on this? Would I maybe want to add some impedence as well? Maybe up to 150ohms or so?

Yes you could try that, this is achieved by buying cables with higher coiling around the wire to increase the resistance offered. However the 50 Ω you find on the Sen HD 555s are quite well placed in the middle.

The question you need to answer is whether or not you will be using this with any small mp3 players. If so, do not increase the impedance, however if your only sound source is the sound card or higher impedance professional equipment, then you yes, you could look into that if you want the most from your headphones.

I would say get a USB DAC, but since gaming is one of your requirement you may want to seriously consider a sound card. You don't need to drop $200 on a sound cars though. I went from a couple modded and tweaked $200+ sound cards to a $75 USB DAC. Sound is better, but you do lose a bit of positional audio for gaming.

Spend what is left on an amp. You don't have to go all out, but a mini3, little dot or used mullet will be fine and not break the bank. The mini3 and mullet are better amps.

When you've done that then consider upgrading the cable on the headphones. It'll give you a noticeable increase in clarity, instrument separation and extension in both highs and lows. The cable is important, but you'll be hard pressed to hear the difference if you haven't a good source and amp.